1. Field of the Invention
This invention relates to a method of fabricating semiconductor devices, more particularly to a method of fabricating epitaxial semiconductor devices.
2. Description of the Related Art
In general, the efficiency of an epitaxial semiconductor device, such as a light emitting diode, a laser diode, a solar cell, etc., is decided primarily by the crystal quality of an epitaxial layer of the epitaxial semiconductor device, which may function based on electroluminescence or photoelectric effects during operation. The epitaxial layer is generally formed by epitaxially growing a semiconductor material on an epitaxial substrate. However, most epitaxial substrates have a poor thermal dissipation property that may result in thermal accumulation during operation. Moreover, the cost of the epitaxial substrate is relatively expensive.
At present, methods of separating the epitaxial layer from the epitaxial substrate and attaching an alternative substrate to the epitaxial layer are conducted for overcoming the above drawbacks. The alternative substrate has a superior thermal dissipation property, and the fabrication cost may be cut down by recycling the expensive epitaxial substrate.
Referring to FIG. 1, a conventional fabrication process for making an epitaxial semiconductor chip includes: (a) forming a sacrificial layer on an epitaxial substrate that is formed of single crystal GaAs in the form of a wafer and that has an etching property_quite different from that of the sacrificial layer; (b) epitaxially forming an epitaxial layer of a semiconductor structure on the sacrificial layer, the etching properties of the epitaxial layer and the sacrificial layer are also quite different such that the two layers have a highly selective etch ratio; (c) adhering a temporary layer on the epitaxial layer opposite to the sacrificial layer; (d) wet etching laterally the sacrificial layer along a periphery thereof in order to separate the epitaxial layer from the epitaxial substrate; (e) adhering a device substrate on the epitaxial layer opposite to the temporary layer using a polymer adhesive and removing the temporary layer; and (f) cutting the epitaxial layer and the device substrate to form a plurality of epitaxial semiconductor chips.
In the step (d) of the conventional fabrication process, the sacrificial layer is wet etched merely laterally along a periphery thereof. The efficiency of the etching may be reduced when the size of the epitaxial layer becomes larger.
As shown in FIG. 2, an epitaxial semiconductor chip 1 made from the aforesaid conventional fabrication process includes a device substrate 11, an epitaxial layer 13 of a semiconductor structure, and a polymer adhesive layer 12 used to adhere the device substrate 11 to the epitaxial layer 13. The polymer adhesive layer 12 of the epitaxial semiconductor chip 1 has a low thermal dissipation property and is easily influenced by the environment. When a device made from the epitaxial semiconductor chip 1 is in use, the internal heat generated by the epitaxial layer 13 may not be dissipated through the polymer adhesive layer 12 and the device substrate 11, and failure of the device may occur.
Referring to FIG. 3, J. J. Schermer et al. (ELSEVIER, Thin Solid Films, 511-512 (2006) 645-653) disclosed an epitaxial lift-off (ELO) process that is used at Radboud University and that is capable of efficiently separating an epitaxial layer 13 from an epitaxial substrate 16. The steps of the lift-off process are similar to the aforesaid conventional fabrication process except that a temporary carrier 17 made from a flexible material and having a size larger than the epitaxial layer 13 is adhered thereto. When a release layer 15 is wet etched along a periphery thereof, the flexible temporary carrier 17 is bent attributed to an external force (Y) applied to the flexible temporary carrier 17. A space 18 between the epitaxial substrate 16 and the epitaxial layer 13 is therefore formed and enlarged so as to increase the contact area of an etchant, thereby resulting in an increase in the etching rate. However, the lift-off method shown in FIG. 3 has a disadvantage that the bending curvature of the flexible temporary carrier 17 involves a higher risk of breaking the epitaxial layer 13.
It is desired in the art to provide a fabrication process for efficiently and safely separating the epitaxial layer from the epitaxial substrate.